Phase lock loops are commonly used in many devices today. Phase lock loops generate a signal having a desired frequency. Accordingly, receivers require phase lock loops to generate signals on which information is modulated. Furthermore, circuits that have timing requirements require clocks that are generated by phase lock loops.
One problem that arises when phase lock loops are used is that spurious signals are generated by the phase lock loop and coupled to other nearby circuits. Such spurious signals interfere with the operation of the other circuits. One example of a system in which spurious signals are induced in circuits that lie relatively close to a phase lock loop is a transceiver having a compactly integrated circuit. In such cases, it is common for phase lock loops to generate energy that is induced into the transmit and receive path of the transceiver. In particular, the receive path of a transceiver is vulnerable to interference from energy that leaks from phase lock loops to the receive path.
One way to deal with the interference from phase lock loops is to spread the energy generated in the output of the phase lock loop over a relatively wide frequency spectrum. By spreading the energy across a broadband frequency spectrum, the amount of energy that is present at narrower frequency bands, such as the frequency at which the transceiver receives signals transmitted by other transceivers, is reduced. In order to evenly distribute the frequency of the phase lock loop over a broad spectrum of frequencies, a sawtooth or triangular spreading signal is used as a spreading signal. That is, a signal that ramps at a constant rate can be used to spread the signal output from the phase lock loop evenly over a relatively broad frequency spectrum.
FIG. 1 is an illustration of a spread spectrum phase lock loop 100. A frequency source 102 provides a reference for the operation of the phase lock loop 100. The frequency source 102 is typically a crystal oscillator or other such stable frequency source. The frequency source 102 is coupled to a phase detector 104. The phase detector 104 has an output that represents the difference in phase between the output of a divider 106 and the frequency source 102. The output of the phase detector 104 is a control voltage that is coupled to a low pass filter 108. The low pass filter 108 is designed to provide the phase lock loop with a sufficiently fast response time that the loop can converge, but not so fast that the loop will overshoot and go into oscillation. The output of the low pass filter 108 is coupled to a voltage controlled oscillator (VCO) 110. The output of the low pass filter 108 attempts to steer the output frequency of the VCO 110 to a frequency that will cause the error signal from the phase detector 104 to be zero. Accordingly, the VCO 110 outputs a frequency that is N times the frequency source, where N is the value by which the divider 106 divides the output of the VCO 110 before providing a signal to the phase detector 104. The output of the VCO 100 is coupled to the output port of the phase lock loop 100. In addition, the output of the divider is fed back to the phase detector 104 to allow the phase detector 104 to produce the control voltage to the VCO 110.
In a spread spectrum phase lock loop such as that shown in FIG. 1, the divider 106 is programmable with a variable value that is input from a triangle wave generator 112. The triangle wave generator 112 loads the divider with a value N that increases in even steps up from a minimum value to a maximum value and then back down again in equal steps. Accordingly, the output of the phase lock loop 100 will start at a frequency that is N times the frequency of the source 102 and increases in frequency with increases in the value of N until it hits the maximum frequency. The frequency output from the phase lock loop 100 then decreases from the maximum to the minimum in equal frequency steps. By making the steps equal the frequency moves smoothly across the frequency spectrum spreading the energy equally over the frequency spectrum.
In some systems that are particularly sensitive to the need to reduce the interference from internal phase lock loops, simply spreading the energy generated by the phase lock loop is not sufficient to mitigate the interference caused by the phase lock loop. In these cases, it is desirable to also provide a filter that can further reduce the power of the interfering spurious signals generated as a byproduct of the phase lock loop. Since the frequency generated by the phase lock loop is spread over a relatively broad frequency spectrum, it is not possible to simply put the interference laden received signal through a filter. Such a filter would impede the passage of the desired received signals as well as the interfering signals generated by the phase lock loop.
Therefore, there is a need for a method and apparatus that can generate frequencies in a way that will not interfere with the operation of circuits near the phase lock loop.